I don't know what Ruricolist means by participle. In "The following are default extensions," "following" is a noun. It is idiomatic in that it appears to be singular, but is followed by a plural verb. Perhaps it is a contraction of "the following things"

By participle I mean that _following_ is the adjective made from the verb _to follow_. In "The following are default extensions," following is not a noun as such but a substantive adjective, which has no number: as in "Blessed are the meek [not the _meeks_ nor the _meek ones_], for they shall inherit the earth."

Actually, lastronin, "followings" as a plural noun suggests two or more groups of "followers" of possibly two or more leaders. E.g., "Both Pat Robertson and Jerry Falwell have large followings. The Reverend Sun Yung Moon's following was notably large in comparison"